diff --git a/rest_framework/validators.py b/rest_framework/validators.py
index 57f8bad53..1c7538073 100644
--- a/rest_framework/validators.py
+++ b/rest_framework/validators.py
@@ -132,13 +132,6 @@ class UniqueTogetherValidator(object):
         """
         Filter the queryset to all instances matching the given attributes.
         """
-        # If this is an update, then any unprovided field should
-        # have it's value set based on the existing instance attribute.
-        if self.instance is not None:
-            for field_name in self.fields:
-                if field_name not in attrs:
-                    attrs[field_name] = getattr(self.instance, field_name)
-
         # Determine the filter keyword arguments and filter the queryset.
         filter_kwargs = {
             field_name: attrs[field_name]